1 Answer
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You've actually found the answer. For the C Major chord, start with the C Major scale and select I, IV, and V (C, F, and G); those are your major scales containing C Major. Then take each of their relative minors (down three half-steps): A, D, and E.

The same thing works for B Major — B Major, E Major, and F# Major plus G# minor, C# minor, and D# minor all contain the B Major chord.

Alternatively, you can think of this in terms of the circle of fifths. C Major and A minor, plus their immediate neighbors on the circle, all contain the C Major chord. This is true for any other chord/key and its relative minor plus their neighbors.